A cylindrical battery is known in which an arc-shaped slit is formed in a circular conductive tab such that, even when a cover of the battery is swollen due to an increase in internal pressure, the tab can be bent at a center thereof by the slit without interfering with a plurality of weld joints (see Patent Document 1).
On the other hand, a thin battery (flat battery) is known in which positive and negative electrode tabs (electrode terminals) have one ends taken out of a laminate package of the battery and the other ends welded by ultrasonic welding to collectors of positive and negative electrode plates of a power generating element of the battery.
In the flat battery, however, there arises a difference in expansion/contraction rate between the electrode tab and the collectors when the electrode tab and the collectors are subjected to load or high temperature during the welding. This can cause wrinkles in the collectors and separations in the weld joints between the collectors and the electrode tab.